Radon is a radioactive gas that is found in natural rock formations. It usually passes harmlessly from cracks in the rock formation into the atmosphere and is dissipated. However, if a structure is built over the formation, the gas can accumulate in the basement. High concentrations of radon gas have been determined to be a health hazard.
Because of the potential hazard from radon gas, inspections of property for the presence of this gas are conducted both to determine if action is needed to protect the occupants or to determine the condition of the building prior to a sale. One method of conducting the inspection is to place previously sealed canisters with charcoal inside them in locations were radon gas is likely to accumulate. The canisters are unsealed so the gas or its by-products can enter the canister and become trapped in the charcoal. After about three days the canisters are collected, resealed and sent to a laboratory where the radioactivity of the charcoal is measured. This measurement is an indication of the presence and concentration of the gas.
Another method of testing for radon gas involves the use of electrets, e.g. Teflon.RTM.. With this method the electret in the form of a film is mounted on a pedestal in a sealable container, the walls of which are conductive. The electret has been electrically charged, e.g., to about 700 volts. If this charged electret is placed in the presence of radon gas, ions created by the radioactivity of the radon or its radioactive by-products cancel the charge on the electret surface. As a result, the average voltage from the electret charge on the electret is decreased.
If a charged electret in its container is left exposed in a location where radon gas is expected, the electret will undergo Loss of Voltage and the container in which it is fixed can subsequently be sealed and the container shipped to a laboratory. The laboratory measures the difference between the voltage before exposure and the voltage after the container was sealed. The voltage difference is an indication of the concentration of radon gas in the location.
The structure of an electret radon gas detector is described in U.S. Pat. No. 5,093,570 of Dorfi et al., which is assigned to the assignee of the present application. This patent shows the electret film mounted on a pedestal. However, it has been found in practice that it is difficult to mount the electret, which is a flexible film, in a taut fashion without wrinkles. Wrinkles and misalignment of the electret film can lead to inaccurate results.